Decorative laminates have been conventionally made by stacking and curing under heat and pressure a plurality of layers of paper impregnated with synthetic thermosetting resins. Normally, the assembly, from the bottom up, consists of a plurality, e.g. three to eight, core sheets made from phenolic resin impregnated kraft paper, above which lies a decor sheet impregnated with melamine resin, over which is provided an overlay sheet which, in the laminate, is almost transparent and provides protection for the decor sheet.
Recently, a revolutionary improvement has been developed which forms the subject matter of Scher et al U.S. Pat. Nos. 4,255,480; 4,263, 081; 4,305,987; 4,327,141; 4,395,452 and 4,400,423 These disclosures are incorporated by reference, and hereinafter reference will be made to Scher et al U.S. Pat. No. 4,255,480. This patent discloses the manufacture of decorative laminates having an ultra-thin protective coating deposited as the uppermost layer of the decor sheet, and which has enhanced abrasion-resistant properties compared with the typical laminate as described above.
According to the process of Scher et al U.S. Pat. No. 4,255,480, the decor sheet is coated with an ultra-thin wet layer of a mixture of an abrasion-resistant hard mineral of particle size exit 20-50 microns, e.g. alumina, in quantities sufficient to provide an abrasion-resistant layer without interfering with visibility, and a binder material for the mineral which binder material, preferably microcrystalline cellulose together with carboxy methyl cellulose, has the properties of withstanding the subsequent laminating condition and which is compatible with the thermosetting resin and which binder material is present in an amount sufficient to bind the abrasion-resistant mineral to the surface of the decor sheet. The coating is then dried at a temperature sufficiently high, at least 140.degree. F. and preferably 180.degree. F., to enhance the bonding of the abrasion-resistant material by the binder material to the face of the decor sheet. Next the coated decor sheet is impregnated with the thermosetting resin. The resultant decor sheet is then used in place of conventional decor sheet in making high and low pressure decorative laminates, without the necessity of using an overlay sheet.
The process of Scher et al U.S. Pat. No. 4,255,480 provides a number of important procedural advantages, besides producing a much better product, and these advantages are outlined in such patent. However, in spite of the considerable product and process improvements which are achieved, the process of U.S. Pat. No. 4,255,480 still requires a double handling of the decor sheet, i.e. it must first be coated and dried, and then impregnated or saturated with the resin, and this results in double losses in the sense that any single handling operation causes inevitable losses, and double handling causes increased losses. In addition, if the ultra-thin abrasion-resistant coating is heavy or the paper particularly dense, as is sometimes the case with certain decor sheets in which a design is printed on highly calendered paper. saturation may be poor which results in poor surface durability, mottling or other defects.
Both the problems with losses and saturating could be solved by a coating method which saturates and coats simultaneously, and therefore such a method would be procedurally desirable.
Of interest as background patents are the early patents to Fuerst U.S. Pat. Nos. 3,373,070 and 3,373,071. The technique of Fuerst is to impregnate the decor sheet first with plain melamine resin, and then apply a thick coating to the surface of the impregnated sheet using a relatively viscous mixture of 2,000-60,000 centipoise. Because the decor paper is already saturated at the time the viscous mixture is applied, it will not be absorbed by the decor sheet, and the viscous coating is dried on the surface. Indeed, Fuerst specifies slow drying which also tends to maintain the surface coating distinct from the decor sheet. The resultant dried surface coating is in essence a formed in situ overlay sheet having a thickness of 2-3 mils and double handling is required.
Two patents of more recent vintage than the Fuerst patents are those of Power U.S. Pat. No. 3,975,572 and Raghava U.S. Pat. No. 4,322,468. While the Power patent is based primarily on the concept of the use of a special acrylic resin-melamine/formaldehyde resin composition which, like the coating of the Fuerst patents, is applied over the already impregnated decor sheet (and indeed after the saturated decor sheet has been dried; see col. 8, lines 47-51), the patentee does mention in passing that "it may be useful to first coat the decorative sheet, dry it and then impregnate with the impregnating composition in some instances" (col. 8, lines 51-55).
Raghava U.S. Pat. No. 4,322,468 relies on a special coating composition of a fully hydrolyzed polyvinyl alcohol modified melamine-formaldehyde resin. The patentee indicates that "the print sheet is first impregnated with a melamine-formaldehyde resin and subsequently dried before the coating operation is commenced" (col. 6, lines 14-17). Abrasive material may be included in the impregnating thermosetting resin, in which case a thin layer of abrasive particles is said to be deposited on the surface of the decor sheet (col 4, lines 25-29).
Besides requiring the utilization of special types of overcoating resins and inability to make the product of Scher et al U.S. Pat. No. 4,255,480, the Power and Raghava methods have three undesirable characteristics. First, double handling is required in most or all instances and two resin systems are required. Second, after impregnation, the core sheet is at least partially dried which has the tendency of creating an entity which, although ultimately united to the special resin later overcoated, retains its distinctiveness as a separate layer. Third, although the abrasive particles can be maintained in suspension in the impregnating resin when using laboratory equipment, and this only by considerable agitation, the problem of the grit settling becomes almost impossible to solve in commercial production equipment.